A good pH level depends on the substance in question. Some substances are meant to be more acidic and others more basic, while some substances should be neutral. For instance, stomach acid has a pH of 1, while pancreatic secretions are basic with a pH of 8.1.
Aquarium keeping is an example of a hobby where knowing the ideal pH is important. Freshwater community aquariums should have a pH of 6.5 to 7.5 because most fish are comfortable in conditions that are either neutral, slightly basic or slightly acidic. Hobbyists can use water conditioners to control pH and ensure that it is within the right range.